High-pressure anneal process for integrated circuits

ABSTRACT

This invention embodies an improved process for annealing integrated circuits to repair fabrication-induced damage. An integrated circuit is annealed in a pressurized sealed chamber in which a forming gas comprising hydrogen is present. Pressurization of the chamber reduces the contribution made by the final anneal step to total thermal exposure by increasing the diffusion rate of the hydrogen into the materials from which the integrated circuit is fabricated. Ideally, the forming gas contains, in addition to hydrogen, at least one other gas such as nitrogen or argon that will not react with hydrogen and, thus, reduce the danger of explosion. However, the integrated circuit may be annealed in an ambiance containing only hydrogen gas that is maintained at a pressure greater than ambient atmospheric pressure.

RELATED APPLICATIONS

This application is a divisional of pending U.S. application Ser. No.09/256,634 filed Feb. 24, 1999; which is a continuation of U.S.application Ser. No. 08/589,852, filed Jan. 22, 1996 and issued as U.S.Pat. No. 5,895,274.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to annealing processes used to repairprocess-induced damage in integrated circuits.

2. Description of Related Art

Integrated circuit devices are typically fabricated in mass on siliconwafers. During the fabrication process, a semiconductor wafer istransformed into a multitude of integrated circuits by subjecting it tonumerous ordered steps, which may include coating with photoresistresin, exposure of the photoresist to electromagnetic radiation using areticle affixed to a highly-precise stepper device, development of thephotoresist to form a mask, oxidation, nitridization, silicidation, wetetching, plasma etching, chemical vapor deposition, plasma-enhancedchemical vapor deposition, sputter deposition, ion implants, annealingsteps for activating and driving implanted ions. A number of these stepscreate residual damage to the material being treated. For example,plasma etches typically leave dangling silicon bonds. In such a case,the silicon surface exposed to the etch comprises many silicon atomswith unfilled orbitals. Another example is the crystal damage wrought byion implants. It is axiomatic that the larger the implanted ion, thegreater the crystal damage. Arsenic, a relatively large ion, isparticularly adept at damaging the silicon crystal structure whenimplanted. Plasma-enhanced chemical vapor deposition processes can alsocreate damage similar to that caused by plasma etches. In addition,silicon dioxide deposited using plasma-enhanced chemical vapordeposition processes tends to trap electrons. The charge buildupresulting from trapped electrons can shift threshold voltage values infield-effect transistors to the point where they will not fully shutoff. Unwanted current leakage through the transistors results.

Following fabrication, but before encapsulation or other packagingsteps, the wafers are annealed at temperatures of about 400° C. in aforming gas containing diatomic hydrogen for approximately 30 minutes inorder to repair the damage caused by the various process steps.Generally, the forming gas contains both diatomic hydrogen and diatomicnitrogen gases. The partial pressure of hydrogen within the forming gasis usually about 20 percent of the total pressure. At annealtemperatures nitrogen functions as an inert gas, as it does not reactwith either hydrogen or the materials from which the circuits werefabricated within this temperature range. Although other inert gasessuch as helium, neon or argon may be used as components of the forminggas, nitrogen is used primarily for cost considerations. In addition,although forming gases having higher concentrations of hydrogen or evenpure hydrogen could be used for the final anneal ambiance, safetyconcerns dictate that hydrogen be diluted to a partial pressure ofaround 20 percent of the total pressure. At elevated temperatures,silicon crystals damaged by implanted ions tend to heal themselves.Although the healing process may be less than perfect, a significantreduction in transistor leakage is observed. During the anneal, thehydrogen gas molecules, on account of their small size, are able todiffuse throughout the circuit and react with any dangling bonds thatare present. Dangling bonds may be present on silicon atoms in substrateregions which have been plasma etched, in polycrystalline silicon layerswhich have been plasma etched, and in silicon atoms in silicon dioxidelayers which have been formed using plasma-enhanced chemical vapordeposition. The diffusion of hydrogen throughout the oxide layersdissipates trapped charges and enhances circuit performance.

During the fabrication process, an integrated circuit is subjected onnumerous occasions to elevated temperature. Generally, the elevatedtemperature is required to effect a necessary step in the fabricationprocess. For example, oxidation of silicon, implant activations, andchemical vapor deposition of silicon dioxides are generally performed attemperatures in excess of 500 degrees centigrade. Metalization steps arealso performed at high temperatures. Although a certain amount ofexposure to elevated temperatures is required both to activate implantedions and to cause them to diffuse within the implanted material, toomuch exposure to elevated temperature will effect too great an overlapand counterdoping of adjacent implants having opposite conductivitytypes and cause source/drain regions of field-effect transistors todiffuse too far into the channel regions. Outdiffusion of dopants intothe channel regions is irreversible, and will result in transistorleakage. Greater outdiffusion will, at some point, completely destroythe functionality of the circuit. The exposure of integrated circuits toheat is analogous in two respects to the exposure of living organisms toionizing radiation. Not only is exposure cumulative, but at someexposure level, the organism will die. Each integrated circuit devicehas an optimum total thermal exposure limit that is generally referredto as the circuit's thermal budget. The budget is calculated by summingall exposures with regard to both time and temperature. Although time isa linear function, diffusion rate does not increase in direct proportionto the increase in temperature. Instead, the diffusion rate increasesgeometrically with increasing temperature.

As device geometries are shrunk for new generations of integratedcircuits, thermal budgets must be lowered by a corresponding amount.Unless the process is modified to reflect these reduced thermal budgets,it will become increasingly difficult to stay within those budgets. Itis a given that final anneals in a hydrogen-containing ambiance forextended periods will become increasing problematic in terms of thermalbudget considerations as device dimensions shrink.

Silicon nitride films,are used extensively as moisture barriers in:integrated circuits. Silicon nitride films are also nearly impermeableto diatomic hydrogen molecules at standard temperature and pressure.This has not posed a problem until fairly recently, as the siliconnitride film is deposited as the final layer in the fabrication process.The hydrogen anneal step is performed prior to depositing the finalsilicon nitride film. However, silicon nitride films are also being usedwith increasing frequency for sidewall spacers and capping layers onfield-effect transistor gates when self-aligned contact etches which arerequired for sub-half-micron geometries. These silicon nitride gatestructures must be fabricated long before the hydrogen anneal step. Thepresence of these silicon nitride structures greatly hampers thediffusion of hydrogen molecules into silicon dioxide layers associatedwith the transistor gates.

Therefore, a new final thermal anneal process is needed that is morecompatible with the limited thermal budgets and the silicon nitride gatestructures associated with the new generations of integrated circuits.

SUMMARY OF THE INVENTION

This invention embodies an improved process for annealing integratedcircuits to repair fabrication-induced damage. An integrated circuit isannealed in a pressurized sealed chamber in which a forming gascontaining hydrogen is present. Pressurization of the chamber reducesthe contribution made by this final anneal step to total thermalexposure by increasing the diffusion rate of the hydrogen into thematerials from which the integrated circuit is fabricated. Ideally, theforming gas contains, in addition to hydrogen, at least one other gassuch as nitrogen or argon that will not react with hydrogen and, thus,reduce the danger of explosion. However, the integrated circuit may beannealed in an ambiance containing only hydrogen gas that is maintainedat a pressure greater than the prevailing ambient atmospheric pressure.Typical anneal temperatures are within a range of 300 to 500° C. forcircuits having aluminum metalization. Higher upper temperature limitsand shorter anneal times may be used for circuits employing metalizationmaterials with higher melting points than that of aluminum. Although theefficiency of anneal operations improves by increasing the annealingpressure above 1 atmosphere, optimum annealing pressures are deemed tobe in the 10 to 25 atmosphere range.

PREFERRED EMBODIMENT OF THE INVENTION

The improved final anneal process, which takes place before theintegrated circuit devices are encapsulated, utilizes forming gas underhigh pressure to increase the diffusion rate of diatomic hydrogen withinthe materials from which the integrated circuits are manufactured. Byperforming the anneal under pressure and increasing the diffusion rateof diatomic hydrogen, the temperature or the duration or both thetemperature and the duration of the anneal process may be reduced.Furthermore, by increasing pressure, anneal operations which werepreviously impractical due to impermeability of certain materials suchas silicon nitride become workable.

The improved final anneal takes place in a sealable anneal chamber inwhich a forming gas comprising hydrogen is present. In a preferredembodiment of the process, the forming gas contains both diatomichydrogen and diatomic nitrogen gases. Due to safety considerations, theoptimum partial pressure of hydrogen with-in the forming gas is about 20percent of the total pressure. If the safety concerns are mitagable,greater partial pressures of hydrogen up to 100 percent of the totalpressure may be employed. In addition, the forming gas may comprise oneor more of other inert gases such as helium, neon and argon. The forminggas within the anneal chamber is pressurized above ambient atmosphericpressure. Although equipment limitations will restrict maximumpressures, process equipment that will handle pressures up to 10atmospheres is generally available. It is believed that even greaterpressures up to 25 atmospheres will further increase the diffusion rateof hydrogen within the materials from which the integrated circuits arefabricated and enable the use of lower anneal temperatures and/orshorter anneal periods. Additionally, it will make practical thehydrogen anneal of circuits having silicon nitride structuressurrounding gate structures. In order to reduce the contribution made bythis improved final anneal step to total thermal exposure, either theanneal temperature or the anneal duration is reduced. Alternately, boththe anneal temperature and the anneal duration can be reduced. Typicalanneal temperatures are within a range of 300 to 500° C. for circuitshaving aluminum metalization, with the optimum anneal temperature beingabout 400° C. Higher upper temperature limits with correspondinglyshorter anneal times may be used for circuits employing metalizationmaterials with higher melting points than that of aluminum. Typically,the anneal temperature is no greater than 80 percent of the meltingpoint of the metal or metal alloy utilized for the metalization.

Although only several embodiments of the improved anneal process aredisclosed herein, it will be obvious to those having ordinary skill inthe art of integrated circuit manufacture, that changes andmodifications may be made thereto without departing from the scope andthe spirit of the invention as hereinafter claimed.

We claim:
 1. A method of affecting a factor that determines at least inpart a threshold voltage of a transistor, comprising: providing asemiconductor device having at least one transistor gate structureincluding a silicon oxide layer and having a film of silicon nitrideused as at least one of a sidewall spacer and a capping layer for afield-effect transistor gate located on a portion thereof, and having asilicon nitride film deposited as a final layer over at least a portionof said semiconductor device; exerting a diffusion pressure on saidtransistor, said diffusion pressure higher than a prevailing ambientatmospheric pressure; exposing said transistor to a hydrogen-containingambiance during said exerting a diffusion pressure; and dissipating acharge trapped in relation to said silicon oxide layer, using hydrogenfrom said hydrogen-containing ambiance.
 2. The method in claim 1,wherein said dissipating a charge comprises diffusing hydrogen from saidhydrogen-containing ambiance throughout said silicon oxide layer.
 3. Themethod in claim 2, wherein: said diffusion pressure on said transistorcomprises exerting a diffusion pressure on said transistor, saidtransistor includes an overlying layer at least partially covering saidsilicon oxide layer; and diffusing hydrogen from saidhydrogen-containing ambiance through said overlying layer to saidsilicon oxide layer.